In organic light-emitting diodes (OLEDs), only part of the generated light is coupled out directly. The rest of the light generated in the active region is distributed among various loss channels, for instance in light guided in the substrate, in a transparent electrode and in organic layers as a result of waveguiding effects, and also in surface plasmons that can be generated in a metallic electrode. The waveguiding effects arise, in particular, as a result of the differences in refractive index at the interfaces between the individual layers and regions of an OLED.
Typically, in known OLEDs, only approximately one quarter of the light generated in the active region is coupled out into the surroundings, that is to say air, for example, while the emission is deprived by virtue of approximately 25% of the generated light being lost as a result of waveguiding in the substrate, approximately 20% of the generated light being lost as a result of waveguiding in a transparent electrode and the organic layers and approximately 30% being lost as a result of the generation of surface plasmons in a metallic electrode. In particular, the light guided in the loss channels cannot be coupled out from an OLED without additional technical measures.
In order to increase the coupling-out of light and thus the emitted light power, measures are known, for example, for coupling out the light guided in a substrate into emitted light. For this purpose, by way of example, films comprising scattering particles, and films comprising surface structures such as microlenses, for instance, are used on the outer side of the substrate. It is also known to provide a direct structuring of the outer side of the substrate or to introduce scattering particles into the substrate. Some of these approaches, for example, the use of scattering films, are already in use commercially and can be scaled up with regard to the emission area in particular in the case of OLEDs embodied as lighting modules. However, these approaches for coupling out light have the major disadvantages that the coupling-out efficiency is limited to approximately 60-70% of the light guided in the substrate, and that the appearance of the OLED is influenced significantly, since a milky, diffusely reflective surface is produced by the applied layers or films.
Furthermore, approaches are known for coupling out the light guided in organic layers or in a transparent electrode. However, to date these approaches have not yet become established commercially in OLED products. By way of example, the document Y. Sun, S. R. Forrest, Nature Photonics 2, 483 (2008) proposes forming so-called “low-index grids”, wherein structured regions comprising a material having a low refractive index are applied to a transparent electrode. Furthermore, it is also known to apply high refractive index scattering regions below a transparent electrode in a polymeric matrix, as is described in U.S. Publication No. 2007/0257608, for example. In this case, the polymeric matrix generally has a refractive index in the region of n=1.5 and is applied wet-chemically. Furthermore, so-called Bragg gratings or photonic crystals having periodic scattering structures having structure sizes in the light wavelength range are also known, as are described in the documents Ziebarth et al., Adv. Funct. Mat. 14, 451 (2004) and Do et al., Adv. Mat. 15, 1214 (2003), for example.
However, by means of such measures, that proportion of the light generated in the active region of an OLED which is converted into plasmons cannot be influenced or even coupled out.